Method for producing a hollow cast component having an inner coating

ABSTRACT

In the method according to the invention for producing a hollow cast component having an inner coating in at least one cavity, the component is cast from a base material. A casting mold, which comprises at least one mold insert for defining at least one cavity is used for the casting. In the method according to the invention, the coating material for the inner coating of the component is applied to at least one mold insert before the component is cast. The mold insert is then removed from the cavity after the casting.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority of the European application No. 04010492.9 EP filed May 3, 2004, which is incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The present invention relates to a method for producing a hollow cast component, in particular a hollow cast turbine blade, having an inner coating on the walls of at least one cavity.

BACKGROUND OF THE INVENTION

Components subject to strong heating may be provided with a coating on their outside in order to protect them against corrosion and/or oxidation in the strongly heating environment. MCrAlY coatings, in particular, are suitable as protection against oxidation and/or corrosion in this context. In the scope of the present description, an MCrAlY coating should be understood as a coating of a metal alloy which comprises chromium (Cr) and aluminum (Al), and in which Y stands for yttrium or a rare earth element and M stands for iron (Fe), cobalt (Co) or nickel (Ni).

Under certain operating conditions, however, the inner walls of hollow cast components operated in a strongly heating environment may also be exposed to temperatures which necessitate protection of the inner walls against oxidation and/or corrosion, so that the component can actually achieve the intended service life.

Turbine components, especially turbine blades, are examples of such hollow cast components subject to strong heating. During operation, turbine blades are exposed to high temperatures and generally have cavities through which the coolant is fed. Under certain operating conditions, temperatures that necessitate protection of the inner walls against oxidation and/or corrosion may occur in these cavities.

To date, the inner walls of hollow cast components have been more or less satisfactorily protected by diffusion aluminizing. These aluminizations reach their limits as the operating temperatures increase, however, and they are virtually ineffective against corrosion.

With conventional methods, it is virtually impossible to apply particularly effective MCrAlY coatings in the sometimes very complicated cavities or interiors of hollow cast components. Spray processes cannot be employed satisfactorily for coating the inner walls of hollow cast components.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a method by which inner coatings, especially MCrAlY inner coatings, can be produced even in complex cavities of hollow cast components.

It is also an object of the present invention to provide an improved hollow cast turbine component.

The first object is achieved by a method as claimed in the claims, and the second object is achieved by a hollow cast turbine component as claimed in the claims.

In the method according to the invention for producing a hollow cast component having an inner coating in at least one cavity, the component is cast from a base material. A casting mold, which comprises at least one mold insert for defining at least one cavity is used for the casting. In the method according to the invention, the coating material for the inner coating of the component is applied to at least one mold insert before the component is cast. The mold insert is then removed from the cavity after the casting.

According to the invention, therefore, the inner coating is produced first and then the component is cast around it. In this way, it is even possible for extremely complex cavities or interiors of components, for instance hollow cast gas turbine blades, to be provided with an inner coating, in particular with an MCrAlY inner coating. The material of the mold insert is in this case selected so that it can be removed from the cavity of the component once it is finished, without taking the coating material with it.

In particular, those materials which can be removed from the cavity by means of a chemical method which does not attack the coating material and the base material of the component are suitable as the material for the mold insert. For example, it is possible to produce ceramic mold inserts which can be removed from cavities of turbine components by means of a suitable lye, which does not attack an MCrAlY coating or the base material of turbine components. Ceramic materials, in particular, are therefore suitable as materials for mold inserts to cast turbine components, for instance turbine blades.

In the method according to the invention, the casting temperature when casting the component is particularly selected so that it is lower than the melting temperature of the coating material, so as to avoid melting the coating material during the casting process and therefore mixing with the base material of the component.

The application of the coating material to the mold insert may, for example, be carried out by means of high-energy spray methods (high-velocity oxygen fuel spraying, abbreviated to HVOF) or plasma spraying (abbreviated to APS).

A hollow cast turbine blade according to the invention has at least one cavity and an inner coating present in the cavity, in particular an MCrAlY coating.

Owing to the inner coating of the hollow cast turbine component, it has improved oxidation and corrosion properties compared with hollow cast turbine components according to the prior art. It can therefore be exposed to higher temperatures.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features, properties and advantages of the present invention will be found in the following description of an exemplary embodiment with reference to the appended figure.

FIG. 1 shows a hollow cast turbine component in a schematic representation.

FIGS. 2-7 show various stages in the production of the hollow cast turbine blade as a highly schematized representation.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 schematically represents a hollow cast turbine blade 1 in a sectional view. The turbine blade 1 comprises a blade body 2 in which, in the present exemplary embodiment, four cavities 3, 5, 7, 9 are formed which, for example, are used to feed cooling air through. The blade body 2 has an outer surface 12, which is provided with an MCrAlY coating for protection against oxidation and/or corrosion. The blade body 2 also has inner surfaces 4, 6, 8, 10 which bound the cavities 3, 5, 7, 9 and form the inner walls of the cavities. The inner surfaces 4, 6, 8, 10 are likewise provided with an MCrAlY coating 14, 16, 18, 20 in order to protect them too against oxidation and/or corrosion.

The method for producing the turbine blade 1 described with reference to FIG. I will be explained below with reference to FIGS. 2 to 7. The method may, however, also be employed for the inner coatings of other hollow cast components.

Firstly, a ceramic mold insert 22 for the casting mold to produce the turbine blade is formed and sintered (FIG. 2). In the next step (FIG. 3) an MCrAlY coating 14 is applied to the ceramic mold insert 22. For example, a high-energy spray method such as high-velocity oxygen fuel spraying (HVOF) or plasma spraying (APS) may be used for this purpose. The thickness with which the MCrAlY coating 14 is applied to the mold insert depends on the thickness with which the inner walls of the turbine blade are intended to be coated.

After the mold insert has been coated, a wax model 24 of the turbine blade is injected onto the mold insert 22 (FIG. 4). The outer mold half 26 of the mold is then applied to the wax model 24. The wax is then burnt off and the mold half 26 is sintered in order to make the casting mold 28 (FIG. 5).

After the casting mold 28 has been made, the turbine blade 1 is cast in a vacuum. The casting of the turbine blade typically takes place at casting temperatures of less than 1500° C. The melting point of the MCrAlY coating 14, on the other hand, is typically more than 1600° C. Melting of the coating 14 during the casting process, and therefore mixing of the coating material with the base material of the turbine blade 1, do not therefore take place. When using coatings other than the MCrAlY coating, the casting temperatures must also be lower than the melting temperature of the coating material, if melting of the coating and mixing with the base material are intended to be avoided.

After the end of the casting, the mold half 26 is removed and the blade 1 is cleaned (FIG. 6). Lastly, the mold insert 22 is leached, i.e. the ceramic mold insert 22 is removed from the solidified turbine blade 1 by means of a lye. Since the MCrAlY coating 14 as well as the blade material are resistant to the lyes being used, the MCrAlY coating 14 remains on the inside of the turbine blade I when the ceramic mold insert 22 is leached (FIG. 7). The turbine blade produced in this way can then be mechanically processed in the conventional way and coated on the outside.

The described method is suitable not only for inner coatings of turbine blades but, in principle, also for inner coatings of other hollow cast components. It is likewise possible to use the method to apply coatings other than MCrAlY coatings. Furthermore, the mold insert need not necessarily be made of a ceramic material. It is merely necessary to ensure that the mold insert can be removed without taking with it the coating applied to the inside of the workpiece.

In respect of the tools for producing the mold inserts and the wax models, care should be taken to correct these by the thickness of the protective layer in order to ensure the specified wall thicknesses of the hollow cast component. 

1-8. (canceled)
 9. A method for producing a hollow cast turbine component having an inner coating in a cavity, comprising: providing a cast mold comprising a mold insert defining at least one cavity to cast a component; applying a coating material to the portion of the cast mold that defines the at least one cavity; casting the component from a base material using the cast mold; removing the mold insert from the cavity of the component after the casting.
 10. The method as claimed in claim 9, wherein a material that can be removed from the cavity by a chemical method and that does not attack the coating material and the base material of the component is used as the material for the mold insert.
 11. The method as claimed in claim 9, wherein a ceramic material is used as the material for the mold insert.
 12. The method as claimed in claim 9, wherein the casting temperature is lower than the melting temperature of the coating material.
 13. The method as claimed in claim 9, wherein the inner coating is an MCrAlY coating.
 14. The method as claimed in claim 13, wherein a high-energy spray method is used for applying the coating material to the mold insert.
 15. The method as claimed in claim 13, wherein the turbine component is a turbine blade or a turbine vane.
 16. The method as claimed in claim 13, wherein at least one of the portions of the cast mold defining a cavity are coated.
 17. A hollow cast turbine component, comprising: a base material; a cavity having an inner surface; and an inner coating material present on the cavity inner surface.
 18. The hollow cast turbine component as claimed in claim 17, wherein the inner coating material is MCrAlY.
 19. The hollow cast turbine component as claimed in claim 17, wherein the component is cast from the base material using a cast mold comprising a mold insert for defining the cavity.
 20. The hollow cast turbine component as claimed in claim 17, wherein a material that can be removed from the cavity by a chemical method and that does not attack the coating material and the base material of the component is used as the material for the mold insert.
 21. The hollow cast turbine component as claimed in claim 17, wherein a ceramic material is used as the material for the mold insert.
 22. The hollow cast turbine component as claimed in claim 17, wherein the casting temperature is lower than the melting temperature of the coating material. 